Double-walled structures for use in ventilation duct systems are well known in the art. In a brochure entitled "LindabSilencer" printed in 1988 and distributed by Lindab Ltd., various double-walled silencers are disclosed. On p. 2 of this Lindab brochure, the general construction of such silencers is described. A typical Lindab silencer comprises an inner tube of perforated thin metal sheet and an outer tube or casing which typically is a helically-wound lock-seam tube of thin metal sheet. The space between the two tubes is filled with sound-absorbing material, such as mineral wool, of different types and densities depending on the field of use. Between the perforated inner tube and the sound-absorbing material, there is a thin fiber layer, the purpose of which is to prevent fibers from the sound-absorbing material from being drawn into the air duct defined by the inner tube. Without such fiber layer, there is a risk that suction forces in the air duct will draw the insulation material into the duct and thereby impair the sound-absorbing effect of the silencer.
Several examples of circular double-walled silencers are shown in the above-mentioned Lindab brochure. Most of these silencers have end pieces at each end which are provided with tubular portions for connection to circular ducts in the ventilation duct system. Other double-walled structures (not shown in the Lindab brochure) are provided with insulation material over their full length, in which case no end pieces are mounted.
For the sake of completeness, reference is also made to Japanese Patent Abstract No. 1,211,608, in which an additional silencer structure is disclosed. Further, British Patent Publication No. 2,102,256 is mentioned as background art, in which FIG. 3 shows still another silencer.
In the prior-art described above, the thin fiber layer for covering the perforated inner tube is provided in the following manner. First, a rectangular piece of fiber material taken from a web-like supply is cut in a first separate operation. The shape of the fiber material piece is adapted to the shape and circumference of the perforated inner tube. Second, the cut fiber material piece is swept about the perforated inner tube, and third the fiber piece is fastened thereon, for instance by means of adhesive. Again, these are separate operations.
The operations are time-consuming and are often carried out manually, which is disadvantageous. The unnecessarily complicated operations for applying the fiber layer increase the total cost of the double-walled structure. Thus, there is a need for providing a new type of double-walled structure which is produced in an improved way.
It is known to use an inner tube in the shape of a helically-wound tube, which can basically be produced in an apparatus of the type disclosed in French Patent Publication No. 1,307,015. The purpose of the apparatus disclosed in this French publication is to provide a helically-wound lock-seam tube covered by a plastic layer on its inside. The inner plastic cover is intended to prevent corrosion on the inside of the tube. Thus, a strip of plastic material is applied on that side of the metal strip which forms the inside of the tube before the strip is supplied to the forming head. In order to provide a continuous plastic cover on the inside of the tube, the apparatus is equipped with a special welding unit for welding the joints of the helically formed plastic strip inside the tube.